There is in the pituitary gland a form of human growth hormone with a molecular weight of 20,000 instead of the 22,000-dalton value seen with the principal form of the hormone. We intend to locate by sequence analysis the exact location of the structural variation. A second structural variant we have isolated has a blocked amino terminus. We have tentatively identified the blocking group as an acetyl. We plan to confirm this result by at least one other procedure. A third variant was found to have enhanced lactogenic properties. This form we have been able to isolate only by cutting from polyacrylamide electrophoresis gels. To have enough material to make structure studies and biological assays possible, we must devise a more efficient isolation procedure. These studies are in progress.